The present invention relates to method and apparatus for detecting the slip of a driven wheel with respect to surface with which the driven wheel is in contact. The present invention more particularly relates to a slip detecting system wherein the rotation of a driven wheel is compared with the rotation of a similar free rolling wheel also in contact with the same surface.
There are a number of situations in which it is desirable to both detect and indicate slip of a driven wheel with respect to a surface against which the wheel runs. Detection of wheel slip is particularly important in connection with motor vehicles, for example, since the loss of traction accompanying the wheel slip may lead to a complete loss of control of the vehicle, resulting in property damage and/or personal injury. If the driver is aware of the slippery nature of the road surface, he can reduce the speed of the vehicle, thereby reducing the danger of loss of control. The majority of experienced drivers can detect extremely slippery road conditions early enough to prevent loss of control of the vehicle. In some situations, however, the road surface may be slippery enough to heighten the risk of high speed travel while still being modest enough to be undetectable by the motor vehicle operator. It would be desirable if some means could be provided for detecting and indicating slippery road conditions to the driver.
There are other situations in which it is desirable to detect the slip of a driven wheel with respect to a surface against which the wheel runs. In agricultural equipment, for example, often heavy implements are driven by belts connected between a driven pulley and a free rolling pulley. If the implement becomes jammed, the driven wheel may begin to slip extensively with respect to the belt, thereby producing substantial friction heat and presenting a serious risk of combustion. Indeed, a number of combines have been completely lost through fires originating in this fashion. It would therefore be desirable to provide some means for detecting and indicating the presence of slip between the driven wheel (i.e., the driven pulley) and the belt.
In the past, some systems have been devised for providing wheel slip detection. In one such system, a special disk containing numerous ridges or dents around its perimeter is affixed to a vehicle wheel whose slip is to be detected. A second, similar disk is affixed to a second, free rolling wheel of the vehicle. Sensors are then disposed adjacent each dented disk for detecting the rotation of the respective wheels. The sensors each provide one output pulse each time a dent passes the sensor. The rate of occurrence of the sensor pulses is therefore directly related to the speed of rotation of the wheel to which the disk is attached. Wheel slip can be detected by comparing the repetition rates of the pulses at the outputs of the two sensors.
Unfortunately, dented disks are difficult and expensive to manufacture, and are likewise difficult to mount upon existing vehicles. The total cost of purchasing and installing the system tends to be so high, in fact, that ordinary drivers are unwilling to spend the money necessary to procure and mount such a system.